1. Field of the Invention
The present invention relates generally to the art of motion picture projection screens, and more particularly to screens having high gain and conservation of polarization for good 3D stereoscopic viewing over a broad viewing angle range created using electromagnetic fields to orient aluminum flakes.
2. Description of the Related Art
The motion picture industry, like many industries, is tradition-bound and technical innovation tends to come from the outside. There are numerous examples of this, such as the introduction of sound, color, widescreen, and 3D. Any change to the existing motion picture infrastructure has to be carefully measured in terms of its economic benefits compared with established industry methods. For any innovation to prevail, the innovation should cooperate with the existing infrastructure by making relatively incremental changes to industry methods.
Certain established methods of manufacture of motion picture screens exist, and as important, specifications and methods exist that must be followed in the motion picture theater in order to obtain the exhibitors' acceptance. Although there are good reasons for exhibitor acceptance of improved motion picture screens, ongoing concerns remain, such as sound system requirements, the ability to clean the screen, the cost of the screen, the means for hanging or installing the screen, and a host of other considerations, not the least of which is the image quality of the screen for both two dimensional and three dimensional (2D and 3D) projection. Motion picture screens preferably have high contrast, an unobtrusive surface, and even illumination across the screen surface viewed from any seat in the house.
Of particular concern in projecting stereoscopic motion pictures is the conservation of polarization. Screens that conserve polarized light frequently have a metallically reflecting surface, which can include non-metal reflectors that behave like metals. Such metallically reflecting surfaces typically include painted aluminum. The properties of that aluminum surface are to a large extent determined not only by the particle size of the aluminum pigment, but also the binder used, and the method of application. Of particular concern in projection of stereoscopic images is that the screen preserves the polarization for stereoscopic image selection. If depolarization occurs, the result will be crosstalk, wherein a portion of the unwanted perspective view is observed by each eye of a user. Crosstalk is undesirable, and detracts from the enjoyment of the stereoscopic movie by reducing the depth effect and causing viewer fatigue.
Certain screen designs employ a lenticular structure for increasing what is termed “screen gain.” In many circumstances, some screen gain is desirable. Estimates are that only one-third of the volume in space in front of a projection screen contains seating, meaning two-thirds of the projected light is wasted or unavailable for the aggregate eyes of the audience. Note that a motion picture screen cannot amplify light, but can only reflect light projected onto the screen.
The more diffusing the surface of the screen, the more uniform the intensity of reflected illumination as a function of angle. Such a screen, in this instance generally referred to as a “Lambertian” screen, is described as a screen with a matte surface, and a perfect matte screen for the purposes of this discussion can be described as one having a gain of 1.0. Such a screen has many advantages, not the least of which is that the screen provides even, enhanced illumination to any seat in the house. In other words, even if an audience member is sitting far to the side, the brightness of the screen remains constant more or less over the entire surface of the screen. A screen with a gain of 1.0 means that every audience member sees an image having the same brightness. However, a screen with a gain of 1.0 wastes a significant amount of light reflected to the ceiling, floor, and sides of the auditorium. A screen with a gain of 1.0 may be desirable, especially for a wide auditorium or one with a balcony or balconies, but for most current theater designs a significant amount of light energy is wasted.
In general, Lambertian screens receive light and scatter light such that the brightness of the surface viewed by an observer is the same regardless of the observer's angle of view. Specular screens are screens which reflect light energy but can be done without equal brightness regardless of angle of view. A perfect specular screen or surface is a mirror.
Theater screens tend to be specular in nature to at least some degree. For a theater screen, light is gathered in the horizontal and reflected toward the seats in the audience where the light is needed. Side light which would be wasted is gathered and reflected to a more appropriate area of the theater, where people are seated. In general, theater screens having a gain much higher than 2.0 may result in hot-spotting. The ultimate hot spot results with a specular screen that is a mirror, exhibiting a very small hot spot.
Screens have been designed that are semi-specular with an eye toward reducing the hot spot. Theater screens are typically curved in a manner designed to make all parts of the screen substantially perpendicular to the viewing audience. Perfect perpendicular geometries cannot be realized, but are approximated by creating virtual curvatures using ribs or lenticules. Ribs or lenticules provide curvature and creating a beneficial viewing effect by shaping the bidirectional reflectance distribution function of the screen. Hot-spotting on a theater screen can be mitigated through screen cylindrical curvature, usually as a concave surface facing the audience.
One screen exhibiting strong gain, conservation of polarization, and spreading the light over well defined angle characteristics is the design used to produce the Kodak Ektalite screen of Chandler. The screen had an apparently unintended benefit of having excellent polarization conservation characteristics. The Kodak Ektalite screen, now out of production, was a concave screen, the inside section of a sphere. The screen had the rectangular shape as required for motion picture and slide projection and was a rigid solid screen, having an aluminum foil coating applied to its concave surface. The aluminum foil coating had a bark-like texture, which served to soften the specular nature of the reflections. The Kodak Ektalite screen had extremely high gain. Radius of curvature of the Ektalite screen was approximately 4.5 times the width of the front surface of the screen.
Problems exist with respect to the Kodak Ektalite-type screen since a solid screen has to be built in place in the theater. Using a Kodak Ektalite-type screen in a theater would significantly depart from exhibitors' current practices. Current screens are relatively easy to ship and assemble. They are rolled into a cylinder, like a rug, for shipment and assembled on a frame with cords attached to the frame pulling on the screen's grommets. The Chandler Kodak Ektalite design generally must be assembled from sections, or installed using some such technique that significantly departs from currently accepted theater screen implementation practice.
In addition, significant issues exist with a solid screen due to speaker placement issues. One of the important practices in the art of motion picture projection is to use a perforated screen, i.e. a screen with a regular pattern of small holes. Loudspeakers are placed behind the screen containing the perforations, allowing sound to pass through the screen. This arrangement gives the audience the sensation that the sound is located in the same direction as the projected image. Theater owners have concerns over a non-perforated screen, as placement of speakers in other locations is perceived to compromise the audience perception of directionality of sound.
A key to the functioning of a polarization-conserving high gain motion picture projection screen is to suitably control the application of the coating to the underlying substrate surface. The reflecting surface itself is the functional part of the screen.
A modern so-called “silver” screen uses aluminum paint coated or painted onto a vinyl surface. The paint or coating material consists of small aluminum flakes held in place by a binder material. One issue with such a design is the random orientation of the flakes, wherein light striking the flakes and the surface is directed in random directions thus exhibiting a drop in gain.
Based on the foregoing, it would be beneficial to provide a design that provides enhancements over screens previously offered. Such a screen must conform to theater owner requirements and preferably exhibit good gain characteristics.